Correction: that should be:
*/5 * * * * if on_ac_power; then nice -n 19 ionice -c3 /usr/bin/backintime --backup-job >/dev/null 2>&1; fi
Correction: that should be:
*/5 * * * * if on_ac_power; then nice -n 19 ionice -c3 /usr/bin/backintime --backup-job >/dev/null 2>&1; fi